The Future of American Fertility

The population fertility rate is an important determinant of the future needs of the population, since the fertility rate largely determines the age structure of the population. A sudden rise in the fertility rate will increase the demand for childcare and schools. A decline in the fertility rate will put more pressure on programs such as Social Security that are directed towards older people and funded on a pay-as-you-go basis.

In "The Future of American Fertility" (NBER Working Paper 14498) researchers Samuel Preston and Caroline Hartnett review the major social and demographic forces influencing American fertility levels.

The authors first distinguish between two total fertility rate (TFR) measures, the period rate and the cohort rate. The more commonly-used period TFR is a snapshot of fertility at one point in time, as it is the number of children a woman would have over her lifetime if at every age she experienced the age-specific fertility rate observed at that point in time. The cohort TFR is average number of children ever born to an actual cohort of women who have completed their childbearing years. The period TFR can be calculated for recent years, because it is not necessary for women to have completed their childbearing in order for their data to be used. However, during a period when the average age at childbearing is rising, as is happening now, the period TFR will be lower than the cohort TFR because it underestimates the extent of childbearing at older ages that will occur in a cohort.

The authors begin their analysis by exploring trends in fertility. The TFR (period or cohort) fell continuously from 1820 to 1975, with the single exception of the post-World War II baby boom. By the late 1970s, the cohort TFR was slightly below the figure of 2.08 children per woman that is needed in order for each generation to exactly replace itself, although the TFR has risen modestly in the last several years

Recent (post-WWII) declines in fertility can be attributed more to changes in family size than to increases in childlessness. For women born in the mid-1930s, families of four or more children were by far the most common family type, accounting for over one-third of women. Thirty years later, this figure had fallen to about 10 percent, while families with two children had become the most common family type.

In recent years, marriage has come to be viewed as less important for childbearing. While only 5 percent of births in 1960 were out-of-wedlock, that figure had risen to 37 percent by 2005. The nature of marriage itself changed during this period as a result of women's improved economic opportunities and greater access to contraception. Women and men became more similar in their activities, with women working more and men doing more housework, These changes no doubt contributed to fertility declines, yet the fact that fertility rates have leveled off and even risen modestly in recent years suggests that bearing children remains a "powerful goal of most American women."

Next, the authors explore the individual-level characteristics associated with fertility in the U.S. Previous research has established that women's educational attainment is negatively associated with fertility in many societies. The authors show that while this is the case in the U.S., the strength of this relationship has been falling over time, even as educational differentials in women's earnings became much steeper.

A second key characteristic is ethnicity. Tabulating the TFR by ethnic group, the authors show that while the TFR of non-Hispanic whites is below that of Hispanics, it would still rank near the top of TFRs of other developed countries. Thus the U.S.'s relatively high TFR is not due exclusively to high fertility among minorities. When the authors estimate the effect of ethnicity on fertility controlling for education and age, they find that non-Hispanic blacks and Hispanics have higher fertility rates than whites. This gap is shrinking for blacks, but rising for Hispanics. Upon closer examination, the rising fertility gap with respect to Hispanics results from a shift in the composition of the Hispanic population away from Cubans and Puerto Ricans, who have relatively low TFRs, and towards Mexican Americans, who have high TFRs.

The authors also explore geographic differences in fertility across the U.S.. They use geographic differences to estimate the effect of male and female wages on fertility. They find that fertility decreases with women's earnings in an area, but rises with men's earnings. As they note, these findings should be interpreted cautiously due to the possibility of selective migration.

The authors briefly consider why U.S. fertility rates are higher than those of all other developed countries. The U.S. may have adapted to the rise in women's wages in ways that better allowed women to combine work with childbearing, for example by introducing longer store hours to increase shopping opportunities or by providing more flexible work schedules. Changes in views on appropriate sex roles within marriage and the necessity of marriage for childbearing may also have kept U.S. fertility rates relatively high. Greater religiosity in the U.S. may also play a role, given that religious practice is positively related to fertility.

The authors conclude by considering the implications of their findings for the future of American fertility. Their estimates suggest that predictable future changes in the ethnic composition and educational attainment of the population will have small, offsetting effects on the TFR, leaving it essentially unchanged. Continued growth in women's relative earnings could lead to a somewhat larger decrease in the TFR, though this estimate may be an upper bound on the true effect. The eventual end of the rise in average childbearing age will lead to an increase in the period (though not cohort) TFR. Finally, the authors caution that elements of "social contagion" such as those that drove the baby boom and bust could add substantial volatility to the future TFR, but these cannot be predicted or accounted for in the approach they have employed.

This research was supported by a grant from the U.S. Social Security Administration administered by NBER.